Transistor Biasing and Small Signal Analysis | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

 Page 1


  
TRANSISTOR BIASING CIRCUITS ANS SMALL SIGNAL ANALYSIS OF BJT 
AMPLIFIERS 9 Hrs. 
Biasing- Types of biasing- DC equivalent circuit of BJT- Load Line-DC and AC Load 
Line Analysis – Hybrid Model of BJT- Hybrid Model Analysis of CE, CB, CC - 
Calculation of Input Impedance, Output Impedance, Voltage Gain, Current Gain 
using hybrid model- Approximate Model of BJT- CE, CB and CC Analysis- Small 
signal equivalent circuit of BJT- Small Signal Analysis of CE, CB and CC. 
 
Biasing 
For proper working of a transistor, it is essential to apply external voltages of correct 
polarity across its emitter-base and collector-base junctions. 
? emitter-base junction is always forward biased  
? collector-base junction is always reverse-biased 
This type of biasing is known as FR biasing 
 
Stability Factor 
 
 
Different Methods for Transistor Biasing 
Some of the methods used for providing bias for a transistor are : 
1. Base Bias or Fixed Current Bias  
It is not a very satisfactory method because bias voltages and currents do not 
remain constant during transistor operation. 
2. Base Bias with Emitter Feedback  
This circuit achieves good stability of dc operating point against changes in ß with 
the help of emitter resistor which causes degeneration to take place. 
3. Base Bias with Collector Feedback  
It is also known as collector-to-base bias or collector feedback bias. It provides better 
bias stability. 
4. Base Bias with Collector And Emitter Feedbacks 
It is a combination of (2) and (3) above. 
Page 2


  
TRANSISTOR BIASING CIRCUITS ANS SMALL SIGNAL ANALYSIS OF BJT 
AMPLIFIERS 9 Hrs. 
Biasing- Types of biasing- DC equivalent circuit of BJT- Load Line-DC and AC Load 
Line Analysis – Hybrid Model of BJT- Hybrid Model Analysis of CE, CB, CC - 
Calculation of Input Impedance, Output Impedance, Voltage Gain, Current Gain 
using hybrid model- Approximate Model of BJT- CE, CB and CC Analysis- Small 
signal equivalent circuit of BJT- Small Signal Analysis of CE, CB and CC. 
 
Biasing 
For proper working of a transistor, it is essential to apply external voltages of correct 
polarity across its emitter-base and collector-base junctions. 
? emitter-base junction is always forward biased  
? collector-base junction is always reverse-biased 
This type of biasing is known as FR biasing 
 
Stability Factor 
 
 
Different Methods for Transistor Biasing 
Some of the methods used for providing bias for a transistor are : 
1. Base Bias or Fixed Current Bias  
It is not a very satisfactory method because bias voltages and currents do not 
remain constant during transistor operation. 
2. Base Bias with Emitter Feedback  
This circuit achieves good stability of dc operating point against changes in ß with 
the help of emitter resistor which causes degeneration to take place. 
3. Base Bias with Collector Feedback  
It is also known as collector-to-base bias or collector feedback bias. It provides better 
bias stability. 
4. Base Bias with Collector And Emitter Feedbacks 
It is a combination of (2) and (3) above. 
5. Emitter Bias with Two Supplies 
This circuit uses both a positive and a negative supply voltage. Here, base is at 
approximately 0 volt i.e. V 
B
 ? 0. 
6. Voltage Divider Bias  
It is most widely used in linear discrete circuits because it provides good bias 
stability. It is also called universal bias circuit or base bias with one supply. 
 
Base Bias with Emitter Feedback  
 
 
 
 
 
 
 
Page 3


  
TRANSISTOR BIASING CIRCUITS ANS SMALL SIGNAL ANALYSIS OF BJT 
AMPLIFIERS 9 Hrs. 
Biasing- Types of biasing- DC equivalent circuit of BJT- Load Line-DC and AC Load 
Line Analysis – Hybrid Model of BJT- Hybrid Model Analysis of CE, CB, CC - 
Calculation of Input Impedance, Output Impedance, Voltage Gain, Current Gain 
using hybrid model- Approximate Model of BJT- CE, CB and CC Analysis- Small 
signal equivalent circuit of BJT- Small Signal Analysis of CE, CB and CC. 
 
Biasing 
For proper working of a transistor, it is essential to apply external voltages of correct 
polarity across its emitter-base and collector-base junctions. 
? emitter-base junction is always forward biased  
? collector-base junction is always reverse-biased 
This type of biasing is known as FR biasing 
 
Stability Factor 
 
 
Different Methods for Transistor Biasing 
Some of the methods used for providing bias for a transistor are : 
1. Base Bias or Fixed Current Bias  
It is not a very satisfactory method because bias voltages and currents do not 
remain constant during transistor operation. 
2. Base Bias with Emitter Feedback  
This circuit achieves good stability of dc operating point against changes in ß with 
the help of emitter resistor which causes degeneration to take place. 
3. Base Bias with Collector Feedback  
It is also known as collector-to-base bias or collector feedback bias. It provides better 
bias stability. 
4. Base Bias with Collector And Emitter Feedbacks 
It is a combination of (2) and (3) above. 
5. Emitter Bias with Two Supplies 
This circuit uses both a positive and a negative supply voltage. Here, base is at 
approximately 0 volt i.e. V 
B
 ? 0. 
6. Voltage Divider Bias  
It is most widely used in linear discrete circuits because it provides good bias 
stability. It is also called universal bias circuit or base bias with one supply. 
 
Base Bias with Emitter Feedback  
 
 
 
 
 
 
 
 
 
Base Bias with Collector Feedback  
This circuit (Fig.) is like the base bias circuit except that base resistor is returned 
to collector rather than to the V 
CC
 supply. It derives its name from the fact that since 
voltage for R
B
 is derived from collector, there exists a negative feedback effect which 
tends to stabilise I
C
 against changes in ß . To understand this action, suppose that 
somehow ß increases. It will increase I
C
 as well as I
C
 R
L
 but decrease V 
C
 which is 
applied across R
B
. Consequently, I
B
 will be decreased which will partially 
compensate for the original increase in ß . 
(i) I
C(sat)
 = V 
CC
 /R
L
 —since V 
CE
 = 0 
 
(ii)  V
C
 = V 
CC
  – (I
B
 + I
C
) R
L
  ? V 
CC
 – I
C
R
L 
Also, V 
C
 = I
B
R
B
 + V 
BE 
Equating the two expressions for V 
C
, we have 
 
I
B
R
B
 + V 
BE
  ? V 
CC
 × I
C
R
L 
 
Since I
B
 = I
C
 /ß , we get 
 
 
Page 4


  
TRANSISTOR BIASING CIRCUITS ANS SMALL SIGNAL ANALYSIS OF BJT 
AMPLIFIERS 9 Hrs. 
Biasing- Types of biasing- DC equivalent circuit of BJT- Load Line-DC and AC Load 
Line Analysis – Hybrid Model of BJT- Hybrid Model Analysis of CE, CB, CC - 
Calculation of Input Impedance, Output Impedance, Voltage Gain, Current Gain 
using hybrid model- Approximate Model of BJT- CE, CB and CC Analysis- Small 
signal equivalent circuit of BJT- Small Signal Analysis of CE, CB and CC. 
 
Biasing 
For proper working of a transistor, it is essential to apply external voltages of correct 
polarity across its emitter-base and collector-base junctions. 
? emitter-base junction is always forward biased  
? collector-base junction is always reverse-biased 
This type of biasing is known as FR biasing 
 
Stability Factor 
 
 
Different Methods for Transistor Biasing 
Some of the methods used for providing bias for a transistor are : 
1. Base Bias or Fixed Current Bias  
It is not a very satisfactory method because bias voltages and currents do not 
remain constant during transistor operation. 
2. Base Bias with Emitter Feedback  
This circuit achieves good stability of dc operating point against changes in ß with 
the help of emitter resistor which causes degeneration to take place. 
3. Base Bias with Collector Feedback  
It is also known as collector-to-base bias or collector feedback bias. It provides better 
bias stability. 
4. Base Bias with Collector And Emitter Feedbacks 
It is a combination of (2) and (3) above. 
5. Emitter Bias with Two Supplies 
This circuit uses both a positive and a negative supply voltage. Here, base is at 
approximately 0 volt i.e. V 
B
 ? 0. 
6. Voltage Divider Bias  
It is most widely used in linear discrete circuits because it provides good bias 
stability. It is also called universal bias circuit or base bias with one supply. 
 
Base Bias with Emitter Feedback  
 
 
 
 
 
 
 
 
 
Base Bias with Collector Feedback  
This circuit (Fig.) is like the base bias circuit except that base resistor is returned 
to collector rather than to the V 
CC
 supply. It derives its name from the fact that since 
voltage for R
B
 is derived from collector, there exists a negative feedback effect which 
tends to stabilise I
C
 against changes in ß . To understand this action, suppose that 
somehow ß increases. It will increase I
C
 as well as I
C
 R
L
 but decrease V 
C
 which is 
applied across R
B
. Consequently, I
B
 will be decreased which will partially 
compensate for the original increase in ß . 
(i) I
C(sat)
 = V 
CC
 /R
L
 —since V 
CE
 = 0 
 
(ii)  V
C
 = V 
CC
  – (I
B
 + I
C
) R
L
  ? V 
CC
 – I
C
R
L 
Also, V 
C
 = I
B
R
B
 + V 
BE 
Equating the two expressions for V 
C
, we have 
 
I
B
R
B
 + V 
BE
  ? V 
CC
 × I
C
R
L 
 
Since I
B
 = I
C
 /ß , we get 
 
 
 
 
 
Base Bias with Collector and Emitter Feedbacks 
In the circuit of Fig., both collector and emitter feedbacks have been used in an 
attempt to reduce circuit sensitivity to changes in ß. If ß increases, emitter voltage 
increases but collector voltage decreases. It means that voltage across R
B
 is 
reduced causing I
B
 to decrease thereby partially off-setting the increase in ß. 
 
Under saturation conditions, V 
CC
 is distributed over R
L
 and R
E
. 
 
Assuming I
B
 to be negligible as compared to I
C
, we get, I
C(sat)
 = V 
CC
/ (R
E
 + R
L
). 
 
Page 5


  
TRANSISTOR BIASING CIRCUITS ANS SMALL SIGNAL ANALYSIS OF BJT 
AMPLIFIERS 9 Hrs. 
Biasing- Types of biasing- DC equivalent circuit of BJT- Load Line-DC and AC Load 
Line Analysis – Hybrid Model of BJT- Hybrid Model Analysis of CE, CB, CC - 
Calculation of Input Impedance, Output Impedance, Voltage Gain, Current Gain 
using hybrid model- Approximate Model of BJT- CE, CB and CC Analysis- Small 
signal equivalent circuit of BJT- Small Signal Analysis of CE, CB and CC. 
 
Biasing 
For proper working of a transistor, it is essential to apply external voltages of correct 
polarity across its emitter-base and collector-base junctions. 
? emitter-base junction is always forward biased  
? collector-base junction is always reverse-biased 
This type of biasing is known as FR biasing 
 
Stability Factor 
 
 
Different Methods for Transistor Biasing 
Some of the methods used for providing bias for a transistor are : 
1. Base Bias or Fixed Current Bias  
It is not a very satisfactory method because bias voltages and currents do not 
remain constant during transistor operation. 
2. Base Bias with Emitter Feedback  
This circuit achieves good stability of dc operating point against changes in ß with 
the help of emitter resistor which causes degeneration to take place. 
3. Base Bias with Collector Feedback  
It is also known as collector-to-base bias or collector feedback bias. It provides better 
bias stability. 
4. Base Bias with Collector And Emitter Feedbacks 
It is a combination of (2) and (3) above. 
5. Emitter Bias with Two Supplies 
This circuit uses both a positive and a negative supply voltage. Here, base is at 
approximately 0 volt i.e. V 
B
 ? 0. 
6. Voltage Divider Bias  
It is most widely used in linear discrete circuits because it provides good bias 
stability. It is also called universal bias circuit or base bias with one supply. 
 
Base Bias with Emitter Feedback  
 
 
 
 
 
 
 
 
 
Base Bias with Collector Feedback  
This circuit (Fig.) is like the base bias circuit except that base resistor is returned 
to collector rather than to the V 
CC
 supply. It derives its name from the fact that since 
voltage for R
B
 is derived from collector, there exists a negative feedback effect which 
tends to stabilise I
C
 against changes in ß . To understand this action, suppose that 
somehow ß increases. It will increase I
C
 as well as I
C
 R
L
 but decrease V 
C
 which is 
applied across R
B
. Consequently, I
B
 will be decreased which will partially 
compensate for the original increase in ß . 
(i) I
C(sat)
 = V 
CC
 /R
L
 —since V 
CE
 = 0 
 
(ii)  V
C
 = V 
CC
  – (I
B
 + I
C
) R
L
  ? V 
CC
 – I
C
R
L 
Also, V 
C
 = I
B
R
B
 + V 
BE 
Equating the two expressions for V 
C
, we have 
 
I
B
R
B
 + V 
BE
  ? V 
CC
 × I
C
R
L 
 
Since I
B
 = I
C
 /ß , we get 
 
 
 
 
 
Base Bias with Collector and Emitter Feedbacks 
In the circuit of Fig., both collector and emitter feedbacks have been used in an 
attempt to reduce circuit sensitivity to changes in ß. If ß increases, emitter voltage 
increases but collector voltage decreases. It means that voltage across R
B
 is 
reduced causing I
B
 to decrease thereby partially off-setting the increase in ß. 
 
Under saturation conditions, V 
CC
 is distributed over R
L
 and R
E
. 
 
Assuming I
B
 to be negligible as compared to I
C
, we get, I
C(sat)
 = V 
CC
/ (R
E
 + R
L
). 
 
 
 
 
Emitter Bias with Two Supplies 
This circuit gives a reasonably stable Q-point and is widely used whenever two 
supplies (positive and negative) are available. Its popularity is due to the fact that I
C
 
is essentially independent of ß.